# Validate Binary Search Tree

## Description

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

* The left subtree of a node contains only nodes with keys **less than** the node's key.
* The right subtree of a node contains only nodes with keys **greater than** the node's key.
* Both the left and right subtrees must also be binary search trees.

**Example 1:**

```
    2
   / \
  1   3

Input: [2,1,3]
Output: true
```

**Example 2:**

```
    5
   / \
  1   4
     / \
    3   6

Input: [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's value is 5 but its right child's value is 4.
```

## Solution

```cpp
class Solution {
public:
    bool isValidBST(TreeNode* root) {
        if(!root) return true;
        return helper(root, numeric_limits<long>::min(), numeric_limits<long>::max());
    }
    
    bool helper(TreeNode* root, long minVal, long maxVal) {
        bool res = minVal < root->val && root->val < maxVal;
        if(root->left) {
            res &= helper(root->left, minVal, root->val);
        }
        if(root->right){
            res &= helper(root->right, root->val, maxVal);
        }
        return res;
    }
};
```


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